Valhalloceras Evans & King 1990

Genus Valhalloceras Evans & King, 1990 Type species

Valhalloceras floweri Evans & King, 1990 from the Olenidsletta Member, Blackhillsian, Floian; by original designation.

Small exogastrically curved conch with subtriangular conch cross section, venter obtusely rounded; lateral sides more acutely curved, and dorsum broadly rounded; siphuncle narrow, subventral with orthochoanitic septal necks; connecting rings thickened and differentiated; siphonal diaphragms present; sutures form weak dorsal and ventral lobes with lateral saddles; shell surface smooth with weak sinus over venter (from Evans & King 1990).

Evans & King (1990) placed this genus within the Phthanoncoceratidae. However, the type species of this family, Phthanoncoceras oelandense Evans & King, 1990, has strongly concave siphuncular segments, thickened connecting rings and relatively long, loxochoanitic septal necks, which is in contrast to the orthochoanitic necks and nearly tubular connecting rings known from Valhalloceras. Therefore, Valhalloceras would by definition be better placed within the Graciloceratidae (see Sweet 1964 for the diagnosis of the Graciloceratidae). However, this placing is ambiguous too, because the figured features of the connecting ring and septal necks of the type specimen of Valhalloceras are from relatively small apical parts of the phragmocone, with details of the connecting ring and septa of later growth stages either not, or poorly, preserved (see Evans & King 1990: pl. 1 figs 8, 10–11). The few specimens of Valhalloceras present in our collection suggest that in parts of the phragmocone from later ontogenetic growth stages the connecting rings are thick and concave (see below). If this is the case (David Evans, pers. com., agrees with this opinion) and the connecting rings in Valhalloceras are concave during later growth stages, the species is intermediate among Oncoceratida, such as graciloceratids with tubular rings and bassleroceratids with concave rings. Hence, Valhalloceras compares closely to Proterocameroceras valhallfonnense sp. nov. with its ambiguous connecting ring shape, a species that is morphologicallly transitional between bassleroceratids, such as Lawrenceoceras, and oncocerids, such as Richardsonoceras Foerste, 1933.